IP Multimedia (IPMM) services provide a combination of voice, video, messaging, data, etc, within the same session. As the number of basic applications and the media which it is possible to combine increases, the number of services offered to the end users will grow, and the potential for enriching inter-personal communication experience will be improved. This leads to a new generation of personalised, rich multimedia communication services, including so-called “combinational IP Multimedia” services.
IMS is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over mobile communication networks. IMS provides features to enhance the end-user person-to-person communication experience through the integration and interaction of services. IMS allows enhanced person-to-person (client-to-client) as well as person-to-content (client-to-server) communications over an IP-based network. The IMS makes use of the Session Initiation Protocol (SIP) to set up and control calls or sessions between user terminals (or user terminals and application servers). The Session Description Protocol (SDP) carried by SIP signalling is used to describe and negotiate the media components of the session. Whilst SIP was created as a user-to-user protocol, IMS allows operators and service providers to control user access to services and to charge users accordingly. Other protocols are used for media transmission and control, such as Real-time Transport Protocol and Real-time Transport Control Protocol (RTP/RTCP).
Within an IMS network, Call Session Control Functions (CSCFs) perform processing and routing of signalling. CSCFs handle session establishment, modification and release of IP multimedia sessions using the SIP/SDP protocol suite. 3GPP TS23.228 describes the logical nodes P-CSCF, I-CSCF, S-CSCF, E-CSCF and BGCF. The S-CSCF conforms to 3GPP TS 24.229 and performs session control services for User Equipments (UEs). It maintains the session state to support the services, and performs the following functions:
it acts as a registrar according to [RFC3261] at registration;
it notifies subscribers about registration changes;
it provides session control for the registered users' sessions;
it handles SIP requests, and either services these internally or forwards them on to a further node; and
it interacts with IMS Application Servers.
The S-CSCF performs SIP routing according to 3GPP routing procedures.
The S-CSCF may further act as a Charging Triggering Function (CTF), according to the principles outlined in the corresponding technical specifications, [TS 32.260] and [TS 32.299] and therefore supports charging for users making use of IMS infrastructure and services. Fulfilling the role of CTF in an IMS network requires a number of actions in the S-CSCF in order to identify the corresponding signalling event triggered by a UE activity. The S-CSCF, when performing the normal routing actions for the SIP signalling events it is handling, determines whether the SIP information represents a chargeable activity and, if so, which type of charging mechanism to apply. In a modification to this architecture, the CTF may implemented in an Application Server with which the S-CSCF communicates over the ISC interface.
The IMS architecture provides for two distinct charging models: offline charging, and online charging. Both online and offline charging involve the sending of accounting information to some centralised charging node for collection and processing.
In order to implement a pre-paid subscription model, online charging is required. Typically, a subscriber purchases a number of credits from an operator. If a SIP session is initiated by a SIP request sent from the subscriber's UE to the S-CSCF, the S-CSCF either makes use of an internal CTF module or contacts an Application Server implementing a CTF. The CTF node detects whether the requested session is subject to a charge or not. If the requested session is subject to a charge, the CTF node contacts an Online Charging Function (OCF) implemented within an Online Charging System (OCS) node in which the subscriber's current credit is stored. If the subscriber's account holds sufficient credit for the SIP request to be granted, the OCF replies accordingly to the CTF, which grants the request. Similarly, if insufficient credit is available, the SIP request may be turned down, or the session may be granted subject to a limited duration.
The CTF (either as a standalone node or as an internal module within the S-CSCF) and OCF implement the online charging functionalities for the IMS architecture. Communication between the CTF and OCF nodes takes place over the Ro interface described in 3GPP TS32.299, and is governed by the DIAMETER protocol. Specifically, the CTF sends DIAMETER Credit Control Request (CCR) messages, while the OCS replies in DIAMETER Credit Control Answer (CCA) messages. These messages are specified in RFC 4006. The OCF uses the accounting information received over the Ro interface to generate Call Data Records (CDRs) and sends these onwards to a centralised billing system.
FIG. 1 illustrates the online charging architecture for an IMS network in the case where the CTF is implemented within a standalone node (AS). As illustrated in the Figure, other nodes within the IMS network may also communicate with the OCF over the Ro interface.
In the case of offline charging within an IMS network, SIP network entities including the S-CSCF that are involved in a session use the Diameter Rf interface to send accounting information, generated by associated CTFs, to a Charging Data Function (CDF). The CDF uses the received accounting information to generate CDRs, and sends these to a Charging Gateway Function (CGF) over the GTP-based Ga interface. The CGF acts as a gateway between the IMS network and the billing domain, communicating with the billing domain over the Bi interface. This offline charging architecture is illustrated in FIG. 2.
FIGS. 3 and 4 illustrate respectively signalling flows for the online and offline charging architectures, where in both cases the charging activity is associated with a call request initiated by a User Equipment (UE) sending a SIP INVITE to the IMS network. It will be appreciated that not all nodes involved in the process are shown. In the case of the online charging mechanism, the S-CSCF exchanges Credit Control Requests (CCRs) and Credit Control Answers (CCAs) with the OCF over the Ro interface, whilst in the case of the offline charging mechanism, the S-CSCF exchanges Accounting Requests (ACRs) and Accounting Answers (ACAs) with the CDF.
Many developing countries deploying IMS architectures rely exclusively on an online charging infrastructure, primarily because the technical infrastructure for setting up an online charging system for an IMS architecture is not as complex and costly as the technical infrastructure needed for setting up an offline charging system. Nonetheless, even operators of online only charging network architectures may want to handle post-paid as well as pre-paid subscribers. In some cases, operators of an offline only charging network may want to handle pre-paid as well as post-paid subscribers.